A liquid crystal display element is incorporated in watches, electric calculators, various measuring instruments, automobile panels, word processors, electronic notes, printers, computers, televisions, etc. The liquid crystal display element utilizes the optical anisotropy and dielectric anisotropy inherent to a liquid-crystalline compound. Examples of known displaying methods include dynamic scattering system (DS type), guest host type system (GH type), twisted nematic type system (TN type), supertwisted nematic type system (STN type), thin film transistor type system (TFT type) and ferroelectric liquid crystal (FLC). Examples of known means for driving these liquid crystal display elements include static driving system, time-division driving system, active matrix driving system and two-frequency driving system.
Among these known liquid crystal displaying systems, STN system and TFT system are most popular because they have the best display properties.
Liquid crystalline compounds for use in STN system and TFT system must meet various requirements. Particularly important among these requirements are the following five requirements:
(1) It has a low viscosity; PA1 (2) It has a wide temperature range within which it assumes a liquid crystal phase, particularly a high clearing point, or causes no reduction of the nematic phase temperature range when incorporated in a liquid crystal composition; PA1 (3) It is stable to external factors such as heat and light; PA1 (4) It has a good solubility with other liquid-crystalline compounds; and PA1 (5) It has an appropriate refractive index anisotropy.
In particular, viscosity is an extremely important factor governing the response speed of a liquid crystal molecule oriented in a liquid crystal panel to an electric field as described in Phys. Lett., 39A, 69 (1972). A liquid crystal composition which can quickly respond to an electric field exhibits a high display quality and is now most desirable.
To provide a liquid crystal composition which can be used in various operating environments, it is necessary that a liquid crystal composition which assumes a liquid crystal phase in a wide liquid crystal temperature range be prepared. In particular, in order to allow the use under high temperature conditions, a liquid crystal composition having a highest allowable temperature of liquid crystal phase, i.e., high clearing point is required.
In order to provide a liquid crystal composition having a high clearing point, a liquid-crystalline compound having a high clearing point is required as a constituent.
Further, a liquid-crystalline compound to be incorporated in a liquid crystal composition, particularly for TFT, is required to be stable to external environmental factors such as moisture, air, heat and light.
In particular, an active matrix liquid crystal display is suitable for advanced data display for televisions or computers and advanced data display inside automobiles and airplanes. However, in the case where a liquid-crystalline compound or liquid crystal composition which does not possess an extremely high specific resistance (high voltage holding ratio) and a good ultraviolet stability is used, as the electric resistance in the liquid crystal panel falls, the contrast falls, thereby causing a problem in the "erasure of afterimage". The high electric resistance of a liquid crystal composition is an extremely important factor governing the life of the liquid crystal composition particularly when it is driven at a low voltage. Therefore, the extremely high specific resistance (high voltage holding ratio) and good ultraviolet stability are extremely important requirements of the liquid-crystalline compound used.
Further, the liquid-crystalline composition is made of, from a few kinds of to twenty or more kinds of, liquid-crystalline compounds to exhibit optimum properties required for individual elements. Accordingly, the liquid-crystalline compound must exhibit a good solubility with other liquid crystal compound(s), particularly at low temperatures under the recent necessity of use in various conditions.
In other words, in order to allow the use within a wide temperature range, particularly at low temperatures, the liquid crystal composition is required to have a nematic phase even at low temperatures. Therefore, the liquid-crystalline compound to be used essentially must exhibit a high solubility with other liquid crystal compound(s) at low temperatures.
In order to realize the high response, it is preferred that a liquid crystal composition having not only a low viscosity, but also a high refractive index anisotropy be used. The thickness of a cell can be reduced by using such a liquid crystal composition having a high refractive index anisotropy while the product of the cell thickness and the refractive index anisotropy value are kept constant. This makes it possible to prepare a liquid crystal display element which can quickly respond while keeping a high display quality.
In order to obtain a liquid crystal composition having a high refractive index anisotropy, of course, a liquid-crystalline compound having a high refractive index anisotropy is required.
In order to realize a liquid crystal composition having better properties, particularly liquid crystal composition for STN and TFT, it has been keenly desired to provide a liquid-crystalline compound which has a reduced viscosity, a wide liquid crystal temperature range (high clearing point), a high chemical stability (extremely high specific resistance, high voltage holding ratio) and a high solubility with other liquid-crystal compound(s) and preferably has a proper refractive index anisotropy.